Ishaan did 12 more push-ups than Tiffany at night. Ishaan did 30 push-ups. How many push-ups did Tiffany do?
Ishaan did 30 push-ups, and Tiffany did 12 fewer than that, so find the difference. The difference is $30 - 12$ push-ups. He did $30 - 12 = 18$ push-ups.